Much of the attention of the room temperature fabricated TFT community has been focused on the search for high-mobility, stable semiconductor materials. But for the manufacture of high quality TFTs, it is also very important to incorporate a suitable gate insulator. The operating voltage can be reduced by increasing the gate capacitance. Some combination of higher gate dielectric constant and reduced film thickness leads to lower voltage operation. The use of a very thin gate insulator is not a suitable approach, given the relatively rough surfaces characteristics of polymer substrate. To ensure pinhole-free coverage, the film should be much thicker than the roughness of the substrate. The use of a high dielectric constant material is therefore the optimum approach for reducing the operation voltage of TFTs. Recently, there have been many efforts to use high-k oxides to reduce TFT operating voltage.
This research composed largely 2 parts. At first, I tried to find out new candidate materials for a gate insulator in TFTs fabricated at room temperature in chapter 3 and 4. Three kinds of Bi-based pyrochlore, $Bi_{1.5}Zn_{1.0}Nb_{1.5}O_7$ (BZN), $Bi_{1.5}Zn_{1.0}Sb_{1.5}O_7$ (BZS) and $Bi_{1.5}Zn_{1.0}Ta_{1.5}O_7$ (BZT), thin films were fabricated at room temperature using RF magnetron sputtering method in chapter 3. Their dielectric and electric properties were characterized to find out if there are any possibilities for application as gate insulator in room temperature fabricated TFTs. In chapter 4, MgO capping layer were introduced to improve the leakage current characteristics of BZN thin films. Thin MgO layer of ~30 nm were formed on BZN thin films by pulsed laser deposition (PLD) method and RF magnetron sputtering system. In later part, using those materials found in chapter 3 and 4 as a gate insulator, pentacene and ZnO based TFTs were fabricated and characterized in chapter 5 and 6, respectively. All fabrication processes were conducted at room temperature.
BZN, BZS and BZT (Bi-pyrochlore material) thin films are successfully deposited at room temperature using RF magnetron sputtering system. BZN, BZS and BZT thin films exhibit relatively high dielectric constant of 56, 26 and 36, respectively, even they deposited at room temperature. Especially 56 of dielectric constant in BZN films represent the highest value of $\varepsilon_r$ obtained so far for 200 nm thick thin films grown at room temperature by sputtering. These high dielectric constant values of three Bi-pyrochlore materials result from their partially nanocrystalline microstructure confirmed by XRD and HR-TEM analysis.
The leakage current density of Bi-pyrochlore thin films is steeply increased at low electric field. The breakdown field of BZN, BZS and BZT thin film is 0.2, 0.15 and 0.3 MV/cm, respectively. This low breakdown field and high leakage current at low electric field (0.5 MV/cm) hinder the use of these films as gate insulators for thin film transistors.
Overall crystallization of BZN films is started at 500 ℃ of annealing temperature. It is well confirmed by XRD pattern and SEM images. After 500 ~ 600 ℃ annealing, many pores are created in BZN thin films, may result from the volume shrinkage during crystallization. This porous structure of 600 ℃ annealed BZN thin films may cause their high leakage current density and relatively low dielectric constant. The dielectric constant increased with increasing annealing temperature, 55 at in-situ deposition to 120 at 600 ℃.
In order to decrease the leakage current of BZN thin films thin MgO capping layers were introduced. MgO thin films were successfully deposited at room temperature using PLD and RF magnetron sputtering. The minimum thickness of MgO films we can get a uniform, smooth surface by PLD method is 30 nm. The both MgO films deposited by PLD and RF sputtering method show the similar properties in surface morphology, I-V characteristics, and dielectric constant - electric field characteristics. 30 nm MgO coated BZN thin films demonstrate significant improvement in leakage current property compared for BZN only films. The measured leakage current density of 30 nm MgO coated BZN layers remained on the order of $~5\times10^{-8}$ A/㎠ without further increase with increasing applied field up to 0.5MV/cm, while the leakage current of BZN only films increase rapidly in the high filed region (>0.3 MV/cm). The dielectric constant of 30 nm MgO coated BZN thin films are reduced to 31.5 from 55 of BZN only films, due to the lower dielectric constant (~10) of the MgO films. The dielectric constant of 31.5 is still high enough to achieve low voltage operation of less than 5V in room temperature TFTs.
Pentacene based TFTs were successfully fabricated on glass substrate at room temperature. In order to operate the TFTs at low voltage, high-k BZN and/or stacked MgO/BZN films were used as a gate insulator. Both TFTs were able to operate low voltage (<10 V). The threshold voltage and field effect mobility of pentacene based TFTs with BZN gate insulator is 0.075 ㎠/Vs and +1.6 V, respectively, while TFTs with stacked MgO/BZN gate insulator has a threshold voltage of -0.5 V and field effect mobility of 0.043 ㎠/Vs.
ZnO thin films were grown at room temperature using RF magnetron sputtering system. Deposited ZnO films had a polycrystalline structure with (002) preferred orientation, even it was deposited at low temperature. The electrical and structural properties of ZnO thin films were optimized to be used as active layer in TFTs.
ZnO based TFTs were successfully fabricated on glass at room temperature. In order to operate the ZnO TFTs at low voltage under 5 V, high-k BZN and/or stacked MgO/BZN films were used as a gate insulator. The ZnO TFTs with stacked MgO/BZN gate insulator present the better performance, namely a higher field effect mobility of ~5 ㎠/Vs, an on/off ratio of $8.2\times10^6$, compared to the TFTs with BZN gate insulator (field effect mobility of ~0.5㎠/Vs, and on/off ration of $1.6 \times10^4$). The subthreshold swing and a threshold voltage of ZnO TFTs with BZN gate insulator is 0.25 V/dec and 1.1V, respectively, while ZnO TFTs with stacked MgO/BZN gate insulator has a subthreshold swing of 0.35 V/dec and a threshold voltage of 2.8 V.
미래의 소자라 불리는 flexible display 의 구현을 위해서는 플라스틱 기판의 사용이 불가피하다. 이를 위해서는 상온에 가까운 낮은 온도에서 트랜지스터 형성이 가능해야 한다. 플라스틱 기판의 특성상 유리전이 온도가 낮아 공정 온도를 높이기가 힘들기 때문이다. 최근 유기반도체나 ZnO 반도체를 이용하여 상온에서 형성 가능하면서도 우수한 특성을 가지는 트랜지스터의 제작에 관한 연구가 많이 진행되고 있다. 그러나 유기박막 트랜지스터나 ZnO 트랜지스터는 그 구동전압이 수십볼트 이상으로 높아 portable device 에 응용하기 어렵다는 단점을 가지고 있다. 이 연구에서는 상온에서 형성되었을 때에도 높은 유전상수와 낮은 누설전류를 가지는 새로운 게이트 절연막을 이용하여 낮은 구동전압을 가지는 유기박막 트랜지스터와 ZnO 트랜지스터를 제작하고자 하였다.
우선 고온에서 형성되었을 때 비교적 높은 유전상수와 낮은 누설전류 특성을 가지는 것으로 알려진 Bi-pyrochlore 계 물질중 세가지인 BZN, BZS 그리고 BZT 박막을 상온에서 증착시켰을 때의 유전 특성과 누설전류 특성을 살펴보았다. RF magnetron sputtering 법을 이용하여 증착된 BZN, BZS, BZT의 유전상수는 각각 56, 26, 36 으로 상온에서 증착된 박막으로는 상당히 높은 값이다. 특히 BZN 의 유전상수 56 은 현재까지 게이트 절연막으로 이용하기 위하여 상온에서 증착된 박막으로는 가장 높은 값으로 저전압 구동 트랜지스터를 구현하기에 충분한 값을 보이고 있다. 각 박막의 미세구조를 투과전자현미경으로 분석한 결과 상온에서 증착하였을 때도 부분적으로 나노 결정립이 생성되어 있는 것을 확인하였는데 이것이 높은 유전상수값의 원인이 되는 것으로 판단하였다. 누설전류 특성을 측정한 결과 각 박막은 3~5V 근처의 낮은 전압에서 절연 파괴가 일어났다. 이는 트랜지스터에 응용하기에는 좋지 않은 특성으로 이를 개선하기 위하여 30 nm 두께의 얇은 MgO 막을 capping layer 로 이용하였다. MgO capping layer 를 이용한 경우 10 V 이상에서도 $10^{-7}$ A/㎠ 이하의 낮은 누설전류 특성을 보였다. 30 nm 의 MgO를 코팅한 BZN 박막의 유전상수는 32로 BZN 의 56 보다 낮은 값을 가지고 있었으나 이 역시 낮은 구동전압의 트랜지스터를 형성하기에는 충분한 값이다.
BZN 박막과 MgO/BZN 박막을 게이트 절연막으로 이용하여 상온에서 유기박막 트랜지스터와 ZnO 트랜지스터를 제작하고 그 특성을 살펴보았다. ZnO 트랜지스터의 경우 BZN 을 게이트 절연막으로 이용한 경우 약 0.5 ㎠/Vs, MgO/BZN 을 게이트 절연막으로 이용한 경우 약 5 ㎠/Vs 의 전계효과 이동도를 나타내었다. MgO/BZN 박막이 BZN 박막보다 낮은 표면 거칠기와 낮은 누설전류 특성을 가지고 있기 때문인 것으로 판단된다. 높은 유전 상수를 갖는 BZN 또는 MgO/BZN 게이트 인슐레이터의 용으로 각 트랜지스터는 모두 5V 이하의 낮은 전압에서 구동 가능하였다.
